Marlo is an English place-name derivative — from the Old English mere-leah (marsh meadow) or as a chosen feminine variant of Marlon. Daughter of Danny Thomas, founder of St. Jude Children's Research Hospital; she serves as National Outreach Director and has led St. Marlo Hampton (Real Housewives of Atlanta). Marlo Stanfield is male (The Wire).
Marlo does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Marlo reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.